Pressurized water nuclear reactors have fuel assemblies in the form of juxtaposed prisms constituting the reactor core in which heat is given off by the fission of pellets of enriched uranium contained in the rods constituting said assemblies.
The nuclear reaction which delivers heat that is transmitted to a cooling fluid constituted by water under pressure is adjusted or stopped by means of control clusters, each constituted by a bundle of twenty-four absorber rods containing boron carbide which absorbs neutrons. The absorber rods of the control clusters are inserted in twenty-four guide tubes of a fuel assembly placed inside the array of fuel rods of the assembly, these guide tubes also constituting part of the framework of the assembly. This framework includes grids fixed to the guide tubes so as to define an array in which the fuel rods and the guide tubes are placed and held.
The control clusters are moved within the core of the reactor in a vertical direction, so as to adjust the nuclear reactor as a function of the depletion of the enriched uranium in the pellets contained in the fuel rods and as a function of variations in energy demand by the electricity network.
The control clusters are moved in the axial direction of the guide tubes of the fuel assemblies by control mechanisms placed on the closure head of the reactor containing the core.
Above the core, the control clusters are guided by guide tubes that form part of upper internal equipment of the reactor and that are disposed in alignment with the fuel assemblies into which the guide clusters are inserted.
The guide tubes of the upper internal equipment of the reactor include guide elements defining guide channels having vertical axes disposed in line with the vertical axes of the guide tubes of the fuel assemblies in which the absorber rods of a control cluster are moved. Each of the guide tubes is disposed vertically above a fuel assembly and serves to guide a control cluster which is inserted into the guide tubes of the fuel assembly.
In the bottom portion of the upper internal equipment of the reactor there is an upper plate of the core which is designed to rest in a horizontal position on the upper portions of the fuel assemblies of the core inside the reactor vessel.
Each of the guide tubes of the upper internal equipment includes guide elements having openings or channels which are in alignment in the axial travel direction of an absorber rod in line with the axis of a guide tube of a fuel assembly.
Some of these guide elements which are disposed in the upper portions of the guide tubes are constituted by guide cards pierced by through openings for passing the absorber rods of the control cluster. These elements are referred to as “discontinuous” guide elements. Other guide elements constitute “continuous” guide elements and they are disposed in the lower portion of the upper equipment, i.e. in that portion of the internal equipment that is adjacent to the upper plate of the core for coming to rest on the fuel assemblies of the core. These continuous guide elements are constituted by sleeves and by split tubes each serving to guide, respectively, two absorber rods or a single absorber rod.
The control cluster has a top portion in the form of a cross-piece to which the absorber rods are fixed via radially-extending arms which are caused to move axially inside the sleeves and the split tubes, these sleeves and split tubes being accessible from the central portion of the guide tube.
The control clusters are moved frequently, such that after an operating cycle, the cladding of the absorber rods in the control clusters can present a certain amount of wear. After being in operation for some length of time, the friction between the cladding of the absorber rods of the control clusters against the guide elements can lead to wear, and more particularly to ovalization of the bores of the guide elements which are interconnected by openings passing the radial arms of the spider assembly or “hub” to which the absorber rods are fixed. The wear phenomenon can be amplified under the effect of the cladding of the absorber rods vibrating inside the guide cards and inside the continuous guide elements, due to the passage of the cooling fluid, particularly during transients, thereby leading to premature wear of the guide elements.
In order to be able to reduce the reactivity of the core of a pressurized water nuclear reactor very quickly, for example in order to perform an emergency stop of the reactor, the control clusters are caused to drop into a fully-inserted position inside the fuel assemblies of the core by releasing the means that connect the control clusters so the displacement mechanisms. In order to guarantee safe operation of the nuclear reactor and a rapid stop of reaction within the core of the nuclear reactor, it is necessary to obtain a drop time for the control clusters that is as short as possible and to maintain this short drop time throughout long-duration operation of the nuclear reactor.
It has been found during operation of certain pressurized water power stations that the drop time of the control clusters can increase significantly, and in some cases reach values above the maximum acceptable threshold for drop time (e.g. for pressurized water nuclear reactors of the 900 electrical megawatt type, a limit time of 2.05 seconds prior to the rods entering a hydraulic damper situated at the bottom of the fuel assembly so as to damp and stop the cluster in the fully-inserted position). This fault is particularly marked for control clusters situated at the periphery of the nuclear reactor core in the vicinity of the cooling water outlet nozzles from the reactor vessel.
One of the possible causes of the drop time of the control clusters increasing while the nuclear reactor is in use is an effect whereby the absorber rods of the clusters are pressed hydraulically against the walls of the guide channels of the guide tubes by the flow of cooling water, this pressing effect increasing with increasing wear in the continuous guide channels of the guide tubes of the upper internal equipment. Wear of the continuous guide channels by the absorber rod clusters produces a kind of “running-in” effect so as to increase the effect of the absorber rods being pressed hydraulically against the guide tubes and it leads to an increase in friction, thereby slowing down the rods, which in turn leads to an increase in the cluster drop time.
With the nuclear reactors currently being manufactured, the guide tubes of the upper internal equipment are subjected in the factory to local enlargement of the continuous guide channels in a plurality of zones (e.g. three zones) along the length of each continuous guide channel. This local enlargement of the continuous guide channel in radial directions, which can be performed by hydraulic expansion, serves to reduce or eliminate the effect of the control clusters being pressed there against due to the wear of the channels while the nuclear reactor is in operation. This ensures that conditions relating to control cluster drop time are maintained throughout the time that the nuclear reactor is in operation.
For nuclear power stations that are already in operation, proposals have been made to replace the worn guide tubes of the upper internal equipment with new tubes, however the cost of such action and the difficulty of performing it, and also the difficulty of transporting, processing, and warehousing the activated guide tubes that have been replaced, make such a technique of replacing guide tubes unattractive for the operator of the nuclear power station.